I'm hoping sigma come out with a new 24-70 f2.8 OS to rival the tamron if its built anything like the 35mm f1.4 its gonna be amazing. doesnt help you right now though. at least when sigma release a lens its available within a month or so not a matter of years... canon i'm talking to you!

I crank the ISO up and still can't get the keeper rate as high at 1/1600 and higher using f4 compared f2.8. 2.8 always wins when both have IS and the ISO is similar stops. Maybe it's more a focusing issue. I certainly came to know the intricacies of the 7D while I had it and used to kind of irritate the BIF guys with the bigger F4 lenses at times when shooting side by side, especially in the golden hour.

That's the point. Sensor size does not affect exposure. Shooting at 1/1600 s, f/2.8, ISO 3200 on APS-C, if you switch to f/4 and keep 1/1600 s and ISO 3200, you'll have half as much light per unit area hitting the sensor. Same would be true with the teeny sensor on my PowerShot S100.

What sensor size does affect is the amount of noise at a given ISO. So compared to 1/1600 s, f/2.8, ISO 3200 on the 7D, you could shoot the FF camera at 1/1600 s, f/4, ISO 6400 and have lower image noise (better IQ) or you could shoot at 1/3200 s, f/4, ISO 12800 and have similar IQ to the 7D shot at f/2.8, ISO 3200 - i.e., a faster shutter speed with the slower lens and equivalent IQ.

Here's a shot with one of one of those 'bigger f/4 lenses' at dusk on an overcast day, FF camera and ISO 6400. Same shot on a 7D would be unusably noisy (and in this case, f/2.8 isn't an option...).

EOS 1D X, EF 600mm f/4L IS II USM, 1/250 s, f/4, ISO 6400

Wow, great photo! It's amazing that is ISO 6400...that setting is completely unusable on the 7D, even in relatively good light. I was just replying to someone the other day who proclaimed that the notion that small pixels increase noise is a "myth"...I'd love them to see this photo. It kind of proves my point...higher noise with smaller pixels is most certainly not a myth....there just isn't enough light per pixel to compare to something like the 1D X (assuming similar framing, anyway...although even with a center crop on the 1D X and a full frame with the 7D, downsampling won't completely mitigate the higher noise of the 7D's smaller pixels.)

I would actually be curious to know if you pushed ISO even higher, 12800, whether that would saturate the sensor more, and with a little post-process pullback, might actually result in less visible noise...

It is a great photo, but (with all due respect to Jon) iris noisy, grainy, and soft. I'm not sure how you don't see that.

Well, first...the existence of noise doesn't change the fact that the photo is well-executed. It is composed well, the bird's head angle is great, the background is nice. The perch is a bit large...but most of the time you don't really have a choice in that matter. Regardless of the technical factors, its a great photo. When I decide whether I like a photo, I first rate the creative aspects, then the technical aspects. If the bird was out of focus, had the wrong DOF, was significantly noisy (this one is not...it's only moderately noisy), was undesirably under or over exposed, etc. then I'd have problems with it. The only issue with the photo at all that I can see is the background noise, and potentially slightly too much of a crop (which is probably where the slight softness comes from.)

I've never cared for excessive sharpness. I like just a touch of soft...no halos, no jaggies. As for the noise...its ISO 6400... My 7D looks that noisy at ISO 1600, so I find that to be pretty amazing. I do think, however, that it would probably actually look LESS noisy at ISO 12800...maybe even ISO 16000. ETTR is often best done by choosing the slowest shutter possible and necessary aperture, then pushing the histogram as far left as possible with ISO. The closer you get the sensor to maximum saturation the better...and if you can't do that by getting more light on the sensor, the next best option is pushing ISO (which is far better than pushing exposure in post most of the time....the one potential exception, at least on Canon cameras, tends to be the top two ISO settings, which use a downstream post-sensor, pre-ADC amplifier that starts to bring in a lot of color noise.)

I think the point was to show how good an image can look at ISO 6400. It's the difference in getting a shot and not, but IMO is more of a sports scenario or where the moment is what counts, rather than a nature photo, unless of course it's come rare specimen, rarely photographed. In any case, it's not the type of photographic problem I'm trying to solve and for which I started this thread. Still interesting, but not very relevant for me outside of comparing it to a ISO 6400 image from the 7D.

A 17-55mm f/2.8 IS lens, will deliver identical geometry to a 27-88mm f/4.48 IS lens on full frame.

depth of field is a function of geometry.

So as the iris or aperture opens up more it allows more light in, but because that light is light that is coming at a higher incident angle it is focused over a greater area if it is behind or infront of the focal plane. So therefore you have a shallower depth of field. See how the red point is larger and therefore will be blurred on the open aperture example?

Here's another example:

People have been misled with crop to full frame conversions for years.

The "35mm equivalent" is what is really important and nothing else. Your images on 35mm equivalent will always look the same no matter what.

From a physics perspective the "35mm equivalent" is capturing identical information. What really matters is the geometry of the light hitting the sensor, and with 35mm equivalent the geometry will always be the same for a given equivalence. Not only that but your flash settings etc will be identical:

Going back to the 35mm equivalent discussion, consider this:

On 7D compared the the 5D Mark III

The sensor is 1.6 x 1.6 times smaller.

35mm equivalent aperture - Multiply F-Number by (1.6 ) . (an f stop is a base 2 log, so even though we have 1.6x1.6 times as much light we take the square root, which is 1.6 to multiply the F number by. (example 2.8 x 1.6 = 4.48, 4.0 x 1.6 = 6.4))

35mm equivalent focal length - Multiply by 1.6

35mm equivalent ISO or light sensitivity - Multiply by (1.6 x 1.6) (bet you haven't heard of that, but if you do the math the 7D's sensor amplifies the signal 1.6x1.6 times more at a given ISO than the 5D3, so even if both say ISO 800, ISO 800 on the 7D is multiplying the pixels the same as ISO 2000 on the 5D Mark III)

The point is that people are often misled by manufacturers changing the geometry of a camera system, particularly putting in small sensors and then claiming otherwise impressive performance numbers which are incredibly misleading because you are measuring them on a different scale.

It's like saying:

I have a million dollars, and then failing to mention these are Zimbabwe dollars worth $20 not, American dollars.

Yes aperture ISO and focal length are fixed numbers, but so are monetary figures, and the most important thing even the most basic dealing of currency has is WHAT currency you're dealing with, and 99% of people require an "equivalent" frame of refference to understand foreign currency or need to do a conversion. Likewise with cameras, geometry (type of currency) is the most important thing when dealing with the performance of a camera system, and the first thing anyone needs to do is bring up a conversion to the local frame of reference, APS-C 35mm, whatever.

To respond to your concern though, there is NO benefit to a 1 stop faster aperture on APS-C sensor vs full frame because they (more than) cancel each other out. You don't stop action any quicker at all, whatsoever, because remember the ISO is skewed too, so all you're doing is just turning up the ISO sensitivity in a roundabout way by. You have been misled into thinking there is anything else going on.

Anyways, get the 24-105mm f/4.0, it will be much better in every way than your 27-88mm f/4.48 IS equivalent lens. Also remember ISO 800 on the 7D is equivalent to ISO 2000 on the 5DIII.

Now of course each lens will have it's own characteristics and each body will likewise have it's own, and the full frame with the 24-105mm will delivery much better image quality as will any full frame body, but if both bodies and lenses were theoretically perfect and had the same resolution these settings would deliver the exact same with completely identical pixels.

I do not "completely misunderstand how full frame works" in that I do understand the relationship of DOF when it comes to just DOF...what is in focus with the same lens, crop vs. FF.

This is most apparent in video, and why FF and the 5D II changed the film world - the ability to blow out backgrounds and create that super thin DOF "look." I have also experienced focus pullers on set having problems with DOF that was too thin, and resulting footage that looked properly focus on the lcd screen but turned out to be soft when viewed at 1080p. Disaster. This is why I would always use the 7D over the 5D. It was more forgiving for focus if the puller was a bit out or the talent moved or swayed or slightly missed the mark, and at f2.8 still allowed shooting in fairly low light.

OTOH, now I'm more experienced with pulling focus myself and in selecting DP's, and plan (hopefully tomorrow) to buy a FF body instead of another crop. Having 2.8 and the option to go with shallow DOF compared to what is available at f4 would be nice. Suffice it to say that I would still prefer a 2.8 IS in a comparable range if it was available. If I want to buy a crop body or dedicated video rig later (like a c100/c300, BMCC or Scarlet), having the 2.8 would also be desirable to get the DOF down and keep the light transmission up. Naturally, IS for video is also a huge benefit in all ways. But I digress. This was a photography question, and I recognize that at some point, I may need to buy a 17-55 2.8 for the crop sensor dedicated rig I buy that will be s35. That lens cannot be used on the 6D.

The rest of the info about light and angles and stuff is all new to me, and I thank you for that. I'm going to read it one more time to let it sink in a bit better

There is one difference though. Bigger pixels mean MORE noise. That is because of the larger surface and volume of the pixels the thermal and readout noise will increase. Therefore if high DOF is a requirement a smaller sensor will likely be the winner considering image quality. Especially when small sensors allow back illumination with very high quantum efficiencies. Small sensors hit the diffraction limit though which will limit their resolution.

I believe the best combo for you is actually the tamron 24-70 vc with 6d/5d (depending on your budget)reason for the tamron: GREAT vc, works extremely well Rather smooth focus rings, obviously not as smooth as canon's offer, but i think its enough. of course, f/2.8 CHEAP PRICE! nearly half of canons 24-70 ii